Leptin, the protein product of the ob gene, may be involved in the regulation of energy balance. Although a clear relationship between energy intake and plasma leptin concentrations has been demonstrated in humans, little is known about the effect of exercise on leptin metabolism. In the present study, we evaluated abdominal adipose tissue leptin production in vivo by arteriovenous balance at rest and during 60 min of moderate-intensity cycle ergometer exercise in five sedentary male subjects. Blood samples were taken simultaneously from an abdominal vein, draining sc adipose tissue, and a radial artery, at rest and every 10 min during exercise. Adipose tissue blood flow was determined by the xenon washout technique. Glycerol release from fat cells can be studied by stable isotope dilution mass spectrometry and is a marker of lipolysis. Plasma leptin concentrations did not change throughout exercise and were the same as the values obtained during resting conditions. The results demonstrate that plasma leptin concentrations and leptin production do not change during an acute bout of moderate-intensity aerobic exercise.